(1) Field of the Invention
The invention relates to a method of fabricating semiconductor structures, and more particularly, to a method of fabricating rim phase shifting masks (PSM) for use in manufacturing integrated circuit devices.
(2) Description of the Prior Art
The continued reduction in the physical dimensions of integrated circuit features challenges the capabilities of current art technologies. To overcome the limitations of the equipment and processes of the current art, new techniques for extending the capabilities of these equipment and processes are being developed. One such enhancement technique is the use of phase shifting masks. Phase shifting masks may be used, for example, to produce integrated circuit feature sizes of the same order of magnitude, or smaller, as the wavelength of light used in the photolithographic processing equipment.
In a typical, non-phase shifting, photolithographic mask, layer features are formed on the mask in an opaque material such as chrome. This chrome layer is formed overlying a transparent quartz substrate. Light is shown through this mask to expose a photosensitive material overlying the integrated circuit as defined by the mask pattern. After the photoresist is developed, the photoresist will reflect a copy or a reverse copy of the mask pattern.
As the pattern features of the circuits approach the wavelength of the exposure light, it becomes very difficult to successfully transfer the pattern features. This is due primarily to diffraction interference that occurs when the openings in the mask are nearly the same dimension as the wavelength of the light. In practice, it is found that the current 248 nanometer lithographic tools cannot reliably create patterns below 0.15 microns.
One approach to extending the capability of the current lithographic technology is the application of the aforementioned phase shifting masks (PSM). In a phase shifting mask, an additional component is added to the chrome and quartz system. Either through the application of an additional transparent layer or through the removal of a portion of the quartz layer to a specific depth, the optical properties are changed in a part of the transparent (not covered by chrome) sections of the mask. Specifically, when light of the lithographic wavelength is shown through the mask, a phase shift is created between light waves that pass through the phase shifting area and the light waves that pass through the non-phase shifting area. By shifting the phase of the light by 180 degrees, nodes, or cancellations of energy, will occur at opaque boundaries between the phase shifted and non-phase shifted areas. This principle is used to create more sharply defined boundary conditions during the photolithographic exposure. Sharper definition leads to improved pattern transfer.
The phase shifted mask principle may be applied using a rim-type PSM, or simply rim PSM, approach. In the rim PSM approach, a phase shifting structure is formed at every opaque (chrome) boundary. A xe2x80x9crimxe2x80x9d of phase shifting structure is used along the entire pattern of the chrome to cause sharper pattern definition.
Referring now to FIG. 1, the construction of a prior art rim PSM mask is illustrated. A transparent substrate 10 is typically comprised of quartz. A phase shifting layer 14 overlies the transparent substrate. The phase shifting layer 14 is another transparent material, such as a spin-on glass, that is deposited to a thickness that will cause a 180 degree phase shift in the lithographic light. An opaque layer 18, typically chrome, overlies the phase shifting layer.
Referring now to FIG. 2, the opaque layer 18 and the phase shifting layer 14 are patterned. The openings formed in the opaque layer 18 and the phase shifting layer 14 correspond to the desired patterned required for the mask layer.
Referring now to FIG. 3, the rim structures are formed in the mask. The opaque layer 18 is etched to pull back the edges or boundaries of the chrome from the edges of the phase shifting layer 14. In practice, this etching involves either an explicit second photolithographic step and anisotropic downward etching of the chrome or an isotropic etching of the chrome using the same masking layer used for the pattern definition shown in FIG. 2. In either case, these rim structures are created by removing additional material from the opaque layer 18 to expose the phase shifting layer 14.
There are drawbacks to the process of the prior art. This process may require two masking steps that must be very carefully aligned. This is both expensive and time consuming. Conversely, if a single mask step is used, the isotropic under etch used to pull back the chrome is difficult to control.
Several prior art approaches concern methods to create phase shifting masks in the manufacture of integrated circuits. U.S. Pat. No. 5,955,222 to Hibbs et al discloses a rim PSM process. This process forms rim structures by both pulling back the opaque edges, as in the prior art, and by etching grooves in the transparent substrate. Only one photolithographic masking step is used in the method. The mask is used for creating the basic opaque layer pattern. Next, a hybrid photoresist layer, composed of both positive tone and negative tone resist, is applied and then exposed through the substrate of the partially formed mask. After resist development, the opaque edges are etched back and the substrate grooves are etched down. U.S. Pat. No. 5,620,817 to Hsu et al teaches a process to form self-aligned rim PSM. A phase shifting layer is first deposited and then patterned. A photoresist layer is then deposited and exposed through the substrate of the partially completed mask. The developed photoresist layer then defines where the opaque layer is anisotropically deposited. The final rim PSM does not use grooves or channels in the substrate. U.S. Pat. No. 5,582,939 to Pierrat discloses a method to form defect-free alternating aperture PSM (AAPSM). An opaque layer overlies a phase shifting layer. The opaque layer is first patterned. Next, the phase shifting layer is patterned using a second masking step. A third mask is used to again etch the phase shifting layer to remove any bump defects. U.S. Pat. No. 5,532,089 to Adair et al discloses a method to form rim PSM. The opaque layer is first patterned. Then, in one embodiment, sidewall spacers are formed on the opaque layer. Grooves are etched into the transparent substrate offset from the opaque layer by the sidewall spacers. The sidewall spacers are then removed, and the edges of the opaque layer are thus spaced back from the grooves of the substrate. In the second embodiment, an opaque layer immediately overlies an etch stopping layer. After the opaque layer patterning, a phase shifting layer is then applied overlying the opaque layer and the exposed etch stop layer. A second masking step is used to pattern the etch stop layer.
A principal object of the present invention is to provide an effective and very manufacturable method of fabricating rim phase shifting masks for use in the manufacture of integrated circuits.
A further object of the present invention is to provide a method of fabricating rim phase shifting masks where the rims are formed by etching notches in the transparent substrate.
Another further object of the present invention is to provide a method of fabricating rim phase shifting masks where the rims are formed by etching notches in a phase shifting layer.
A yet further object of the present invention is to provide a method of fabricating rim phase shifting masks using a single photolithographic masking step.
Another yet further object of the present invention is to provide a method of fabricating rim phase shifting masks using a single dry etching step.
In accordance with the objects of this invention, a new method of fabricating a rim phase shifting mask is achieved. An opaque layer is provided overlying a transparent substrate. A resist layer is deposited overlying the opaque layer. The resist layer is patterned. The opaque layer and the transparent substrate are etched. The resist layer masks this etching. The opaque layer is etched through during this etching. Notches, as in the form of sidewall trenches, are thereby etched into the transparent substrate at the edges of the opaque layer. These notches will cause a phase shift in incident light relative to incident light passing through regions in the transparent substrate adjacent to the notches. During this etching, an overetch is performed to remove any mask defects in the transparent substrate.
Also in accordance with the objects of this invention, a new method of fabricating a rim phase shifting mask is achieved. A transparent substrate is provided. A phase shifting layer is deposited overlying the transparent substrate. An opaque layer is deposited overlying the phase shifting layer. A resist layer is deposited overlying the opaque layer. The resist layer is patterned. The opaque layer and the phase shifting layer are etched. The resist layer masks this etching. The etch must have a high etch selectivity of the opaque layer to the phase shifting layer, which, in turn, must have a high selectivity to the transparent substrate. The opaque layer is etched through during this etching. Notches, as in the form of sidewall trenches, are thereby etched into the phase shifting layer at the edges of the opaque layer. These notches will cause a phase shift in incident light relative to incident light passing through regions in the phase shifting layer adjacent to the notches. During this etching, an overetch is performed to remove any mask defects in the phase shifting layer.
Also in accordance with the objects of this invention, a new method of fabricating a rim phase shifting mask is achieved. A transparent substrate is provided. A transparent etch stop layer is deposited overlying the transparent substrate. A phase shifting layer is deposited overlying the transparent etch stop layer. An opaque layer is deposited overlying the phase shifting layer. A resist layer is deposited overlying the opaque layer. The resist layer is patterned. The opaque layer and the phase shifting layer are etched. The resist layer masks this etching. The opaque layer is etched through during this etching. The transparent etch stop layer prevents etching into the transparent substrate. Notches, as in the form of sidewall trenches, are thereby etched into the phase shifting layer at the edges of the opaque layer. These notches will cause a phase shift in incident light relative to incident light passing through regions in the phase shifting layer adjacent to said notches. During this etching, an overetch is performed to remove any mask defects in the phase shifting layer.